2012 BFY Abstract Detail Page

We will demonstrate a relatively simple, affordable and highly visual experiment to explore molecular spectroscopy by measuring the laser-induced fluorescence (LIF) spectrum of the iodine molecules at room temperature. Iodine is a uniquely suited seed molecule for LIF measurements since it conveniently absorbs about 20,000 lines in the 490- to 650-nm visible region of the spectrum and serves excellent example of displaying discrete vibrational bands at moderate resolution and rotational structure at high resolution.

The apparatus consists of a diode laser 532 nm (or a laser pointer), an iodine cell, and a handheld spectrometer. We will scrutinize the LIF spectrum about the potentials associated with the vibrational states of the diatomic molecules and assign spectral lines based on the transition probability between vibrational levels, build vibrational energy level diagram and tabulate Deslandres table, evaluate the harmonic and anharmonic characteristics of two states and thereof the merits of the harmonic approximation for the molecular oscillator, and finally extract the molecular constants such as dissociation energies of the molecular potentials.

In this workshop, the rotational structure is not seen at a resolution of about 0.2 nm, a common limit for commercial ultraviolet-visible spectrometers, but the vibrational features can be easily discerned in the measurement. At the end of the workshop, we will discuss how to determine rotational inertia and rotational temperature if a higher resolution spectrometer is available.

Experimental explorations in instructional laboratories of molecular spectroscopy are instrumental not only in educating students about the quantum mechanical phenomenology ingrained into the microscopic structure of matter, but also familiarize them with the germinal scientific puzzles and revolutionary answers that historically led to the discovery of quantum mechanics. Thus, a great deal of effort was directed in our department toward maintaining an advanced laboratory course focused on spectroscopy of atoms and molecules, for a diverse and solid education of our upper-level physics majors.